Cryogenic system for producing ultra-high purity nitrogen

ABSTRACT

A process for further purification of nitrogen gas, thereby producing ultra-high purity nitrogen which is substantially free of hydrogen, oxygen and carbon monoxide, wherein the nitrogen gas is contacted with a metal-containing adsorbent at a temperature of 150 K. or less.

FIELD OF THE INVENTION

This invention relates generally to the production of ultra-high puritynitrogen and, more particularly, to the production of ultra-high puritynitrogen using a process operating at a cryogenic temperature.

BACKGROUND ART

Consumers of nitrogen in the electronics industry typically requireultra-high purity nitrogen which contains less than 1 part-per-billion(ppb) of any contaminant such as oxygen, hydrogen and carbon monoxide.Concentrations of these substances in nitrogen obtained from aconventional cryogenic air separation plant are typically in the rangeof about 0.5-2 parts-per-million (ppm). Oxygen, which has a higherboiling point than nitrogen, is almost completely removed by thecryogenic distillation. However, since the boiling point of carbonmonoxide is very close to that of nitrogen, and that of hydrogen is muchlower, most of the carbon monoxide present in the feed air to thecryogenic air separation plant is also present in the nitrogen productstream from the plant, and the hydrogen concentration in the nitrogenproduct stream is about double that in the feed air.

Removal of these contaminants is typically carried out using aconventional adsorption process following the cryogenic air separationprocess. However, such a system is disadvantageous because of the largesize of the adsorption vessels needed to carry out the purification.

An alternative to the use of conventional ambient temperature adsorptionprocesses for producing ultra-high purity nitrogen is the upstreamoxidation of hydrogen and carbon monoxide to water and carbon dioxide,respectively. These oxidation products are then removed in a molecularsieve prepurification system prior to the cryogenic air separation. Thisoxidation is typically carried out as a catalytic process. A majordisadvantage of this oxidation process is that it requires hightemperatures, increasing the energy requirements, and hence the cost ofthe entire process. Another disadvantage is that the oxygen remaining inthe product nitrogen stream must be removed by another means, usually aseparate cryogenic distillation process, which adds further to the costof the overall process.

Accordingly, it is an object of this invention to provide an improvedsystem for producing ultra-high purity nitrogen.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent to one skilledin the art upon a reading of this disclosure, are attained by thepresent invention which is:

A cryogenic adsorption process for producing ultra-high purity nitrogen,said process comprising contacting nitrogen gas containing one or moreof hydrogen, oxygen or carbon monoxide impurities with ametal-containing adsorbent at a temperature of 150 K. or less, andproducing ultra-high purity nitrogen which is substantially free ofhydrogen, oxygen and carbon monoxide.

As used herein the terms "cryogenic adsorption" and "cryoadsorption"mean an adsorption process carried out at a temperature of 150 K. orless.

As used herein, the term "column" means a distillation or fractionationcolumn or zone, i.e., a contacting column or zone wherein liquid andvapor phases are countercurrently contacted to effect separation of afluid mixture, as for example, by contacting or the vapor and liquidphases on a series of vertically spaced trays or plates mounted withinthe column and/or on packing elements which may be structured packingand/or random packing elements. For a further discussion of distillationcolumns see the Chemical Engineers' Handbook fifth edition, edited by R.J. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section13, The Continuous Distillation Process. A column may include a topcondenser wherein vapor is condensed for column reflux.

Vapor and liquid contacting separation processes depend on thedifference in vapor pressures for the components. The high vaporpressure (or more volatile or low boiling) component will tend toconcentrate in the vapor phase whereas the low vapor pressure (or lessvolatile or high boiling) component will tend to concentrate in theliquid phase. Partial condensation is the separation process wherebycooling of a vapor mixture can be used to concentrate the volatilecomponent(s) in the vapor phase and thereby the less volatilecomponent(s) in the liquid phase. Rectification, or continuousdistillation, is the separation process that combines successive partialvaporizations and condensations as obtained by a countercurrenttreatment of the vapor and liquid phases. The countercurrent contactingof the vapor and liquid phases is adiabatic and can include integral ordifferential contact between the phases. Separation process arrangementsthat utilize the principles of rectification to separate mixtures areoften interchangeably termed rectification columns, distillationcolumns, or fractionation columns. Cryogenic rectification is arectification process carried out, at least in part, at temperatures ator below 150 degrees Kelvin (K.).

As used herein, the term "indirect heat exchange" means the bringing oftwo fluids into heat exchange relation without any physical contact orintermixing of the fluids with each other.

As used herein, the terms "upper portion" and "lower portion" of acolumn mean those portions respectively above and below the midpoint ofthe column.

As used herein, the term "top" of a column means that section of thecolumn above the internals, e.g. trays or packing, of the column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram of a preferred embodiment ofthe invention in which the cryoadsorption process is integrated with acryogenic air separation plant.

FIG. 2 is a process flow diagram of another preferred embodiment of theinvention in which the cryoadsorption process is employed to purifynitrogen drawn from a storage tank.

FIG. 3 is a process flow diagram of a system for regenerating adsorbentin the embodiment illustrated in FIG. 2.

DETAILED DESCRIPTION

In the method of this invention, nitrogen gas, which contains one ormore of hydrogen, oxygen or carbon monoxide impurities at the ppm level,is contacted with an adsorbent at a temperature of 150 K. or less.Preferably, the temperature is no greater than about 120 K., and mostpreferably the temperature is in a range from 80-100 K. The adsorbentadsorbs substantially all of hydrogen, oxygen or carbon monoxide whichmay be present in the nitrogen gas, resulting in an ultra-high puritynitrogen product which is substantially free of hydrogen, oxygen andcarbon monoxide. Carrying out the adsorption process at a cryogenictemperature allows nitrogen gas produced in a cryogenic air separationsystem to be withdrawn from the separation system and directly purifiedwithout undergoing a heating step prior to the adsorption step, thusreducing operating costs. Nitrogen that is being held in a storage tankmay also be purified with the method of this invention. In thisembodiment, nitrogen from a cryogenic storage tank is contacted with theadsorbent at or near the storage temperature without preheating. Thecryoadsorption system of this invention also enables the use of muchsmaller adsorption vessels than those necessary with conventional highertemperature or ambient temperature systems thus reducing the capitalcosts of the purification system.

Suitable adsorbents for the method of this invention which are capableof efficiently adsorbing hydrogen, carbon monoxide and oxygen atcryogenic temperatures are shown in Table 1 and include adsorbents thatcontain nickel, copper, palladium, or iron. The preferred adsorbents arenickel-containing adsorbents, and the most preferred adsorbent isnickel(II) oxide on an alumina support. Preferably, the adsorbent iscontained in the form of an adsorbent bed in a vessel of suitablecapacity for the required quantity of adsorbent.

In a preferred embodiment of this invention, adsorbents are regeneratedby heating in an atmosphere of hydrogen and ultra-high purity nitrogenat a temperature greater than 120° C., preferably greater than 200° C.The most preferred composition of the atmosphere used for regenerationof the adsorbent is about 1% hydrogen, by volume of the total mixture,in ultra-high purity nitrogen. The hydrogen reacts with the carbonmonoxide and oxygen on the surface of the adsorbent to form methane andwater, respectively. The methane and water are more weakly bound to theadsorbent and thus can be removed easily from the surface of theadsorbent by the nitrogen stream. Regeneration is performed at varyingintervals depending on the capacity of the adsorbent vessels and theconcentrations of hydrogen, carbon monoxide and/or oxygen in thenitrogen used as a feed for the process. Typically, multiple vessels areemployed to allow regeneration of one or more vessels while one or morevessels are in use for producing ultra-high purity nitrogen.

FIG. 1 is a process flow diagram showing the cryoadsorption nitrogenpurification system of this invention integrated with a cryogenic airseparation plant. Referring to FIG. 1, feed air in piping 1 iscompressed in compressor 2 and carbon dioxide, water and somehydrocarbons are removed by prepurifier 3. Hydrogen and carbon monoxide,which may be in the feed air, are not removed by the prepurifier becausethey are not adsorbed by the molecular sieve materials of theprepurifier at the compressor discharge conditions. The cleaned airstream in piping 4 is then cooled to cryogenic temperatures by indirectheat exchange in a heat exchanger 5 against return streams. The cooledair stream in piping 6 is fed into a cryogenic rectification column 7,wherein the feed air is separated by cryogenic rectification intonitrogen gas and oxygen-enriched liquid. The nitrogen gas has arelatively low concentration of oxygen, typically less than 1 ppm. Thedistillation process shown is meant to represent a generic distillationprocess that produces nitrogen having a low level of oxygen. Thedistillation process, per se, does not affect purifier design andconfiguration. Therefore, any process which removes oxygen to low levelsfor the production of nitrogen may be used in conjunction with thisinvention. In a process that does not produce nitrogen at cryogenictemperatures, the product stream is cooled before carrying out theprocess of this invention. Ultra-high purity nitrogen can be added backinto the top of the distillation column to improve distillationefficiency, and thereby decrease the oxygen content in the nitrogen gasproduced by the column.

Cryogenic nitrogen gas leaves the distillation process through piping 8,bypass piping 9 and bypass valve V1 to the cryogenic purifier 12, or tooxygen analyzers 10 where its oxygen content is measured prior toentering the cryogenic purifier to prevent high-oxygen excursions.Another portion 30 of the nitrogen gas is passed into top condenser 31wherein it is condensed and returned to column 7 as reflux stream 32.Oxygen-enriched liquid is passed from the lower portion of column 7 instream 33 into top condenser 31 wherein it is vaporized by indirect heatexchange with the aforesaid condensing nitrogen gas and from when it isremoved from the system in waste stream 34.

The cryogenic purifier 12 is shown in FIG. 1. A summary of the valves(V1-V18) follows:

    ______________________________________                                        V1        Control valve for bypass 9.                                         V2        Back-up liquid piping 21.                                           V3        Vent valve for piping 11.                                           V4        Valve for back up piping 21.                                        V5        Valve for purified nitrogen product exiting                                   purifier.                                                           V6        Purge valve for piping 9 or 13.                                     V7        Isolation valve to Bed 12A used to isolate from                               product stream for regeneration cycle.                              VB        Isolation valve to Bed 12A used to isolate from                               product stream for regeneration cycle.                              V9        Regeneration vent for Bed 12A.                                      V10       Product valve from Bed 12A for piping 22.                           V11       Isolation valve to Bed 12B used to isolate from                               product stream for regeneration cycle.                              V12       Isolation valve to Bed 12B used to isolate from                               product stream for regeneration cycle.                              V13       Regeneration vent for Bed 12B.                                      V14       Product valve from Bed 12B for piping 22.                           V15       Valve for regeneration cycle to allow purifier                                product flow 14.                                                    V16       Valve for regeneration cycle to allow pure                                    hydrogen to blend with product flow 15.                             V17       Valve for regeneration of Bed 12A.                                  V18       Valve for regeneration of Bed 12B.                                  ______________________________________                                    

Two oxygen analyzers are used for redundancy. The nitrogen vapor streampasses through piping 11 and enters the cryogenic purifier 12 wherehydrogen, carbon monoxide and any remaining oxygen are removed to be ata concentration of about 1 ppb or less. Valve V3 is a vent valve forpiping 11. The purified product stream leaving the purifier in piping 13is warmed to ambient temperature in heat exchanger 5, then recovered.Valve V6 is a purge valve for piping 9 or 13.

The purifier 12 of this invention comprises two beds, 12A and 12B, thatcontain a metal-containing adsorbent, e.g. at least 5% nickel balancedalumina adsorbent material, preferably 10-30% nickel balanced aluminaadsorbent material, and most preferably, 20% nickel balanced aluminaadsorbent material. The purified product comes from either Bed 12A orBed 12B. There are a pair of isolation valves on each purifier bed toisolate it from the streams carried in piping 11 or 21 for regenerationor a possible shut-down. Isolation valves V7 and V8 are used on 12A; andvalves V11 and V12 on 12B. One bed 12A is regenerating while the otherbed 12B is adsorbing.

Regeneration is accomplished by mixing about 5% of ultra-high puritynitrogen product from piping 14 and valve V15 with hydrogen from tank27, passing through valve V16 and piping 15 to make a mixture containingabout 1 volume percent hydrogen which is heated in heater 16 to atemperature of no less than 120° C., and preferably at least 200° C.Typically the bed to be regenerated is heated to at least 120° C. priorto introducing the 1% hydrogen blend. The hot stream passes throughpiping 17 and valve V17 to purifier bed 12A, or valve V18 for 12B,heating the respective adsorbent bed and releasing any adsorbedsubstances. The spent regeneration stream is vented through valve V9 andpiping 18 from 12A, or through valve V13 and piping 18 from 12B.

The regeneration process requires no less than 120° C., preferably atleast 200° C. and hydrogen to react with the adsorbed carbon monoxide toform methane, and to react with oxygen to form water. The methane andwater along with the hydrogen are then easily desorbed. The regenerationcycle is 24 hours, at which time the beds are switched to replace theadsorbing bed with a regenerated bed. Regeneration can be done every 24hours or every few days, depending on the vessel capacity. Regenerationflow is countercurrent to the adsorption flow.

The cryogenic purification system of this invention may also be used toremove 0.5-1 ppm oxygen and 1-2 ppm carbon monoxide from nitrogen storedand transported in liquid form. Nitrogen is typically stored in liquidform to meet usage needs when the cryogenic rectification plant is notin service. Stored backup liquid in tank 19 is heated to ambienttemperature using a vaporizer 20, passing through valve V4, piping 21and valve V2 to the cryogenic purifier. Stored liquid product typicallycontains little hydrogen because hydrogen boils off in transport and inthe liquid storage tank. Carbon monoxide loading, which is identical ateither ambient or cryogenic temperatures, dictates vessel size in thecryogenic adsorber. Therefore, the cryogenic purifier size may be usedfor either process or liquid nitrogen purification.

Because the purifier is at a cryogenic temperature when process nitrogenis passing through it, backup nitrogen leaving the purifier from bed 12Aor bed 12B is initially cold. If it is required to warm the productnitrogen, it is passed through valve V10, or valve V14, respectively,and piping 22 to an electric heat glycol/water bath 23. The warmedstream passes through piping 24 to the filter skid 25. The streamspassing through either piping 13 or 24 are treated in a filter skidassembly 25 to remove fine particles before the product stream exitsthrough piping 26 for recovery.

In addition, at the same mass flow rate, the volume of vaporized liquidnitrogen that would pass through the purifier would be greater than thevolume of cryogenic nitrogen passing through the regenerator duringnormal operation. Pressure loss through the purifier when vaporizedliquid is purified is about four times that observed with process vapor.However, the pressure available in the liquid storage tank is typicallyrelatively high. Therefore, this increased pressure drop will not causeproblems.

FIG. 1 illustrates a particularly preferred embodiment of this inventionwherein a portion of the ultra-high purity nitrogen produced by thecryoadsorption system is returned to upper portion, preferably to thetop, of the cryogenic rectification column 7 in piping 28. This refluxoperation improves the efficiency of the air separation system andlowers the oxygen content of the nitrogen gas produced by the cryogenicrectification column. The ultra-high purity nitrogen passes into the topcondenser for the production of reflux. If desired, condensed nitrogengas from top condenser 31 may be removed in piping 29 and recovered asultra-high purity liquid nitrogen.

Table 1 contains data from laboratory characterization work done onidentified cryoadsorption materials at 87° K. (-183° C.). The criterionfor selecting a cryoadsorbent material is that the material adsorb themaximum number of moles of carbon monoxide, oxygen and hydrogen per moleof metal in the cryoadsorbent. The best cryoadsorbing materialidentified from the laboratory work was the Crosfield nickel catalystHTC-500™ (available from Crosfield Catalysts, Chicago, Ill.).

                                      TABLE 1                                     __________________________________________________________________________    SOURCE/ADSORBENT                                                                          COMPOSITION                                                                             H/M                                                                              CO/M                                                                              O/M                                                                              Comments                                      __________________________________________________________________________    METADYNE (Elma, NY)                                                                       Tungsten Sponge                                                                         -- <0.01                                                                             -- >700° C. to regenerate                 Union Carbide (Danbury,                                                                   9% CuO-Y Zeolite                                                                        --  0.23                                                                             --                                               CT)                                                                           Crosfield/HTC-500 ™                                                                    20% NiO/Al.sub.2 O.sub.3                                                                0.01                                                                              0.09                                                                              0.02                                            BASF (Mt. Olive, NJ)/                                                                     CuO/ZnO/Al.sub.2 O.sub.3                                                                -- <0.01                                                                             -- Loading based on 40% Cu.sup.+                 R-3-12 ™                                                                   Union Carbide/OC-112 ™                                                                 50% Cuo; Mno.sub.2 /SiO.sub.2                                                           -- <0.01                                                                             <0.01                                            Carus (Ottawa, IL)/                                                                       75% MnO.sub.2 ; 15%                                                                     -- <0.01                                                                             <0.01                                                                            Loading based on 75% Mn.sup.+3                Carulite-300 ™                                                                         CuO/Al.sub.2 O.sub.3                                              Degussa (Tulsa,                                                                           0.5% Pd/Al.sub.2 O.sub.3                                                                -- 1.1 --                                               OK)/E-221 ™                                                                United Catalyst                                                                           Fe/Al.sub.2 O.sub.3                                                                     -- <0.01                                                                              0.01                                                                            O.sub.2 loading based on 80% Fe.sup.+2        (Louisville,                                                                  KY)/C12-4-02 ™                                                             Engelhard (Iselin,                                                                        18% Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                  -- <0.01                                                                             <0.01                                            NJ)/CR-0211 T 5/32" ™                                                      Engelhard/Silica 2351 T                                                                   90% Silica/Al.sub.2 O.sub.3                                                             -- --  <0.01                                            1/8" ™                                                                     Engelhard/Co-0138 E                                                                       30% Co/Silica                                                                           --  0.02                                                                             <0.01                                            1/16" ™                                                                    Various     5A Mol. Sieves                                                                          --  0.04                                                                             --                                               __________________________________________________________________________

In Table 1, H/M, CO/M, O/M are mole to mole ratios of atomic hydrogen,carbon monoxide and oxygen respectively to the given metal.

In another preferred embodiment of this invention, a stand-alonecryoadsorption system is employed to produce ultra-high purity nitrogenfrom standard grade nitrogen such as from a liquid nitrogen storage tankor a liquid trailer. A process flow diagram is shown in FIG. 2. Thecryoadsorption system of this invention is used to purify nitrogen gascontaining one or more of oxygen, hydrogen or carbon monoxide at levelsof 0.1-10 ppm to levels of 1 ppb or less by cryogenic gas phasechemiadsorption.

As shown in the process flow diagram in FIG. 2, liquid nitrogen iswithdrawn from tank 101, flows through piping 102 and passes through avaporizer 103, or through valve 104 bypassing the vaporizer. Thevaporized nitrogen is maintained in either case at a temperature of 150K. or less, preferably from 80-100 K., and most preferably at about 90K. The nitrogen flow in piping 105 is either at full product flow as agas or, as a liquid, at about 1-10 gallons per hour to the top of thecatalyst bed where it flash vaporizes. The flash vaporization maintainsthe cryogenic refrigeration of the catalyst bed when not in use. Thecryogenic high purity nitrogen vapor flows through the catalyst bed 106where at least one of the impurities (oxygen, hydrogen, and carbonmonoxide) is chemiadsorbed to 1 ppb or less. The vessel 107 thatcontains the catalyst bed has only an inlet and outlet port for flowand/or catalyst handling. The vessel employs cryogenic insulation tomaintain the cryogenic temperature. It is vertically supported. Theultrahigh purity nitrogen vapor remains at cryogenic temperature, andexits from the bottom outlet of the bed through piping 108. Theultra-high purity nitrogen is warmed to ambient temperature in vaporizer109, its pressure is regulated by pressure regulator 110, and it passesthrough a particulate filter skid 111 to eliminate any particulatematter before being recovered.

The preferred nickel catalyst bed has the capacity to purify cryogenicnitrogen vapor for 30 days without the need to regenerate. The bedpreferably is large enough to purify nitrogen from a liquid nitrogenstorage tank or a liquid trailer for 30 days, and to allow for a lessfrequent regeneration interval, e.g., 6-12 months. For example, for aplant that will produce 180,000 cubic feet per hour (CFH) nitrogen, therequired amount of catalyst is at least 500 pounds, but no greater than5,000 pounds.

The cryopurifier shown in FIG. 2 does not have permanent regenerationcapability. Regeneration is done using auxiliary equipment on oroff-site to regenerate the catalyst. The required equipment and setup isillustrated in FIG. 3. Regeneration is accomplished by mixing about 5%of the ultra-high purity nitrogen product stream, passing through piping201 and valve 210 near ambient temperature, with a hydrogen supplystream passing through piping 202 and valve 211 to make a regenerationstream in piping 203 comprising nitrogen and about 1 volume percenthydrogen. The regeneration stream enters a static mixer 204 to ensuregood mixing. A mixer is not absolutely necessary to carry out theprocess successfully. The regeneration stream then passes through piping205 and enters a heater 206, where it is heated to not less than 120°C., preferably at least 200° C. As the heated stream exits the heaterthrough piping 207 it passes through a shut off valve 212 and enters thecatalyst adsorbent bed 208. The adsorbent bed is heated by the streamand releases any adsorbed substances. The spent regeneration stream isvented through the bed and piping 209 to exhaust.

The regeneration process requires a temperature of at least 120° C.,preferably at least 200° C., and a gas stream containing hydrogen toreduce the adsorbed carbon monoxide to methane, and to reduce oxygen toform water. The methane and water, along with the hydrogen mixturestream, are then easily desorbed. The regeneration cycle, includingheating and cooling sequences, can be from 24 hours up to 2 weeks,depending on the vessel capacity. Regeneration flow is countercurrent tothe adsorption flow.

Although the invention has been described in detail with reference tocertain preferred embodiments, those skilled in the art will recognizethat there are other embodiments of the invention within the spirit andthe scope of the claims.

What is claimed is:
 1. A cryogenic chemiadsorption process for producingultra-high purity nitrogen, said process comprising contacting nitrogengas containing one or more of hydrogen, oxygen or carbon monoxideimpurities with a metal-containing adsorbent containing at least one ofnickel, copper, palladium and iron, at a temperature of 150 K. or less,and producing ultra-high purity nitrogen which is substantially free ofhydrogen, oxygen and carbon monoxide.
 2. The process of claim 1, whereinthe adsorbent contains nickel.
 3. The process of claim 2, wherein theadsorbent is nickel(II) oxide mechanically supported on a substratecomprising alumina.
 4. The process of claim 1, further comprisingregenerating the adsorbent at a temperature no less than about 120° C.5. The process of claim 4, wherein the adsorbent is regenerated incontact with a mixture of hydrogen and ultra-high purity nitrogen. 6.The process of claim 1, wherein the nitrogen gas is drawn from acryogenic rectification column.
 7. The process of claim 1, wherein thenitrogen gas is drawn from a liquid nitrogen storage tank.
 8. Acryogenic chemiadsorption process for producing ultra-high puritynitrogen, said process comprising contacting nitrogen gas containing oneor more of hydrogen, oxygen or carbon monoxide impurities with ametal-containing adsorbent containing at least one of nickel, copper,palladium and iron, at a temperature of 150 K. or less, and producingultra-high purity nitrogen which is substantially free of hydrogen,oxygen and carbon monoxide, wherein the nitrogen gas is drawn from acryogenic rectification column and wherein a portion of the ultra highpurity nitrogen is introduced into the upper portion of the cryogenicrectification column.
 9. The process of claim 8, further comprisingrecovering liquid ultra-high purity nitrogen from the cryogenicrectification column.
 10. A cryogenic chemiadsorption process forproducing ultra-high purity nitrogen, said process comprising contactingnitrogen gas containing one or more of hydrogen, oxygen or carbonmonoxide impurities with a metal-containing adsorbent containing atleast one of nickel, copper, palladium and iron, at a temperature of 150K. or less, and producing ultra-high purity nitrogen which issubstantially free of hydrogen, oxygen and carbon monoxide, wherein thenitrogen gas is drawn from a liquid nitrogen storage tank and is flashvaporized prior to the contact with the adsorbent.